The production of integrated circuits ("ICs") in semiconductor materials generally involves bringing different fluids into direct contact with the semiconductor materials. For example, in the production of ICs in silicon (Si) and/or gallium arsenide (GaAs) wafers, it is common to bring gases or liquid chemicals into contact with the wafers for purposes such as: (i) cleaning, (ii) etching, (iii) application of resists, etc.
It is well known that metal impurities present in the gases or liquid chemicals used in processing semiconductor materials have a profound detrimental effect on manufacturing yield and the performance of integrated circuits. These impurities, present in starting gases or liquid chemicals, or generated by equipment failures, react with the semiconductor materials to form electron hole recombination centers which degrade the semiconductor materials and any integrated circuits produced therefrom.
In processing semiconductor materials, metal impurities are difficult to monitor and control, due to the very low threshold concentrations required for IC yield degradation. The range of surface contamination of most interest in the semiconductor industry is between 10.sup.8 atoms/cm.sup.2 and 10.sup.12 atoms/cm.sup.2. The problem is even more complex due to the wide variability of the contaminating power of different chemicals used in IC processing. For example, NH.sub.3 OH with 0.1 ppm of iron (Fe) will leave about 10.sup.12 Fe atoms/cm.sup.2 on a silicon surface, which is extremely damaging, while hydrogen fluoride (HF) with the same Fe concentration will cause only marginal contamination on the level of 10.sup.9 Fe atoms/cm.sup.2.
Thus, as a quality control measure during the production of ICs, it is common to measure the contamination of the fluid medium(s) which contact the semiconductor materials for metal impurities which degrade the quality of the semiconductor material(s). For example, it is common to measure the contaminants in the fluid medium(s) before and after processing to ensure that undue contamination has not occurred. One method of measuring the extent of the contamination of the fluids is by spectroscopy. However, this method of testing is not suitable for continuous monitoring the fluid because a sample of the fluid must be removed periodically from the system for testing. Meanwhile, the wafers processed by the contaminated fluids before the contamination is detected become contaminated themselves. These contaminated wafers may undergo at least partial processing into chips--processing that is wasteful because the wafer is already contaminated.
One common method for determining the contamination of the processed wafers measures a property of the semiconductor known as the minority carrier diffusion length "L". The minority carrier diffusion length indicates the effective distance that excess carriers diffuse through a semiconductor during their lifetime. Excess carriers in a semiconductor tend to redistribute due to a diffusion phenomenon which equalizes the carrier concentration. The fewer the recombination sites, the farther the excess carriers can diffuse before they recombine. In other words, longer measured diffusion lengths correspond to fewer recombination sites. This diffusion process is controlled by the mobility of the excess minority carriers ".mu." and their lifetime ".tau.". The diffusion length L is a parameter combining these two factors, and in the simplest case has the form: ##EQU1## where k is Boltzman's constant, T is the temperature in Kelvin, and q is the elementary charge.
As discussed above, metal contaminants in silicon wafers act as recombination centers which reduce the minority carrier lifetime .tau.. By measuring the diffusion length L, the concentration of the contaminants may then be determined by using the relationship N.sub.c .apprxeq.C.tau..sup.-1 (where N.sub.c is the concentration of heavy metal contaminants, and C is a constant depending on the individual impurity).
A common, nondestructive technique for measuring the diffusion length L takes advantage of the process by which light impinging upon a semiconductor surface may be absorbed and produce excess carriers (holes and electrons) if the energy of the incident photons, "hv", is above the semiconductor energy band gap "E.sub.g ". As a result of this photogeneration and diffusion process, a certain number of electron-hole pairs reach the proximity of the surface and become separated by the electric field of the surface-space charge region to produce a photovoltaic effect refered to as "surface photovoltage" (SPV). Measurement of the surface photovoltage can thus be used for the determination of the minority carrier diffusion length L, in turn for the determination of the lifetime .tau., and hence for a determination of the concentration of the heavy metal contaminants N.sub.c.
Some prior techniques for determining the diffusion length from the surface photovoltage rely on a procedure known as the "Constant Magnitude Surface Photovoltage" (CMSPV) technique, the principles of which were proposed by Goodman in "A Method for the Measurement of Short Minority Carrier Diffusion Lengths in Semiconductors," J. Appl. Phys. Vol. 33, p. 2,750, 1961; subsequently adopted as the ASTM standard ANSI/ASTM F-391-78 p. 770, 1976 and discussed in U.S. Pat. No. 4,337,051.
The characteristic steps of the foregoing techniques are to measure the photovoltage and the photon flux at several wavelengths (.lambda..sub.1 . . . .lambda..sub.i) (corresponding to photon energies (hv.sub.1 . . . hv.sub.i)); vary the magnitude of the photovoltage by adjustment of the incident light intensity or photon flux (.phi.) to produce a constant photovoltage; measure the corresponding photon fluxes (.phi..sub.1 . . . .phi..sub.i); and then plot the photon flux values versus the reciprocal absorption coefficient .alpha..sup.-1 of the semiconductor sample at the various photon energies. This plot is then linearly extrapolated to determine the intercept along the reciprocal absorption coefficient axis to obtain the minority carrier diffusion length L (i.e., L=-.alpha..sup.-1 where I=O). Thus, in the CMSPV techniques, the diffusion length is determined from the corresponding values of the photon fluxes (.phi..sub.1 . . . .phi..sub.i)) required to maintain the constant magnitude SPV signal V.sub.1 =V.sub.2 =V.sub.3 . . . .
A more recent technique for determining the diffusion length L is disclosed in the applicant's U.S. Pat. No. 5,025,145. According to this technique, an induced photovoltage is first measured for different photon fluxes to assure linearity of photovoltage versus photon-flux. Next, using light with constant photon flux of the value within the linear photovoltage range, the photovoltage is measured for a series of selected photon energies and those photovoltage values which monotonically increase with the photon energy are plotted as a function of the reciprocal absorption coefficient corresponding to the given photon energies. The minority carrier diffusion length is determined by extrapolation to find the reciprocal absorption coefficient at zero photovoltage (i.e., L=-.alpha..sup.-1 where .phi./.DELTA.V=0). The values outside of the monotonical range are rejected from the analysis, which eliminates interference from the surface effects and assures an accurate determination of the diffusion length. This method determines diffusion length directly from the surface photovoltage measured in the different incident photon energies.
Additional background information on sensing metal contamination of semiconductor wafers may be found in J. Lagowski, et al., "Non-Contact Mapping of Heavy Metal Contamination for Silicon IC Fabrication", Vol. 7, Semiconductor Science & Technology, A185-A192 (1992), a copy of which is attached hereto as Exhibit A and is made part of this disclosure.
Several methods exist to take SPV measurements. One method is the contact electrode, wherein a semitransparent material such as indium tin oxide (ITO) is placed in contact with the silicon surface being illuminated. Another method is to capacitively couple the SPV to an electrode. One specific type of capacitive coupling electrode is the non-contact electrode. With a non-contact electrode, the dielectric is air, which allows the wafer being tested to remain untouched by the electrode. The Lagowski, et al. paper, Exhibit A, has a specific discussion of capacitive coupling and non-contact sensing of SPVs at pages A187-88.
In both of the SPV techniques discussed above, a semiconductor wafer is used as a sensor, and the surface photovoltage is measured at the surface that was in direct contact with the fluid medium. In the applicant's experience, the wafer used in such techniques would have a thickness which is considerably greater than its bulk diffusion length--typically, at least twice as thick.
However, in these thick-wafer SPV methods, the configuration is such that the measured parameter, i.e., the minority carrier diffusion length, is sensitive to the recombination centers in the bulk, but is not sensitive to surface contaminants. Therefore, to test for contamination, the contaminants, which typically form on the surface of the wafer, are annealed into the wafer at a high temperature in an additional processing step. For example, a test wafer is included with the wafers that were being processed. After a various number of processing steps are complete, the process is shut down and the test wafer is annealed and analyzed to determine the amount of contamination the batch has sustained. Therefore, these methods of determining the extent of contamination are believed not suitable for constant monitoring of fluids used in semiconductor processing.